1. Field of the Invention
The present invention relates to an impeller of a turbo fluid machine using liquid such as water as a working fluid, a blade used for the impeller, a method and an apparatus for manufacturing the impeller. More particularly, the present invention relates to a less expensive method for manufacturing the impeller of the turbo fluid machine realized by plate working of the impeller irrespective of the form and the fluid type.
2. Description of the Related Art
The turbo fluid machine includes a centrifugal compressor using gas such as air as the working fluid in addition to the centrifugal pump using liquid such as water as the working fluid. An exemplary turbo fluid machine of those described above is disclosed in JP-A No. 7-167099.
The centrifugal pump using water as the working fluid, for example, will be described with respect to the main components by referring to FIG. 1. The centrifugal pump includes impellers 6, 7, a casing 1, an axis of rotation 2, and a motor (not shown) Each of the impellers 6 and 7 has a structure with plural blades 5 interposed between a boss 3 and a shroud 4. They are rotated by the axis of rotation 2 to apply energy to the fluid. That is, rotation of the impeller applies the centrifugal force to water accommodated from an inlet 8. The flow direction of the fluid is optimized by a guide vane attached to an outlet port of the impeller.
An axial-flow pump of the centrifugal pump has a feature that the blade of the impeller has a torsion with respect to the flow path direction in order to efficiently convert the pump rotational energy into the kinematic energy of the fluid.
Basically, the centrifugal compressor has substantially the same structure as that of the centrifugal pump. For example, in a multistage turbocompressor, the impellers 6 and 7 each having plural blades 5a and 5b attached to the respective bosses 3 are mounted on the same axis 2 as shown in FIG. 2. In the multistage turbocompressor, each blade of the respective impellers has the different shape. The blade used for the compressor has the blade surface designed in accordance with the linear element as substantially the straight line.
In the method for manufacturing the turbo fluid machine, the impeller is produced by casting, and then machining. If the high profile accuracy of the blade is required, the impeller as a whole may be subjected to the machining so as to be manufactured. If the blade of the impeller has the three-dimensional torsion, the press forming using the three-dimensional forming die exclusively tailored to the respective blades may be employed.
The casing is produced through the plate working method in which the blade formed of the press formed steel plate is welded to inner and outer cylinders each formed by subjecting the steel plate to the roll forming.
The blade of the compressor as one of the existing turbo fluid machines is subjected to the machining after the casting. However, in the aforementioned process, the large diameter parts may lower the material yield. The action for solving the aforementioned problem is required to be taken. The press forming using the three-dimensional forming die especially tailored to the respective blades has a large ratio of the die cost to the manufacturing cost upon plate working of the impeller. The similar problem may occur in manufacturing of the blade and casing of an mixed flow pump.
In the method for manufacturing the casing, the use of the sheet processing machine instead of the three-dimensional forming die for producing the guide vane may suppress the cost for the die. However, it is impossible for the generally employed sheet metal processing machine to subject the blade to the three-dimensional torsion in principle. Accordingly, the method is not suitable for manufacturing the impeller. It has been demanded to realize the plate working of the impeller at the low cost has been demanded as the essential task of the present invention.
There are problems with respect to subject the blade to the three-dimensional forming. That is, when the blade with the three-dimensional torsion is press formed with the upper and the lower dies, as the blank material is not restrained between the dies at the initial stage of the forming where the blank material and the die partially contact with each other, the blank material, thus is likely to misalign. In the generally employed method for manufacturing the blade, the blank material which contains the margin to be larger than the finished blade in consideration of the displacement is press formed. Then the closest region to the blade surface with the finished shape is cut from the formed material. However, the blank material for forming the blade to be used under the specific environment, for example, the seawater pump is expensive. It is therefore demanded to suppress the material yield. Suppression of the material misalignment upon press forming, thus, is the essential task to be realized by the present invention. Vibration of the impeller caused by oscillation in the profile accuracy of each of the blades may cause noise in operation of the turbo fluid machine. It is therefore essential to establish the assembly accuracy of the impeller.